vvEPA
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lates and soluble materials are carried
downcurrent. Substantial dilution may
take place in the discharge pipe before
the mud is discharged. Drilling mud
concentration, measured as total
suspended solids, percent transmit-
tance, or concentration of particulate or
soluble mud-associated metals,
decreases rapidly with distance from
the rig. Background values for total
suspended solids and soluble/
particulate metals concentrations are
usually reached within 100 to 1,000
meters (m) downcurrent of the
discharge. Using ultratrace techniques
for particulate barium, it is possible to
trace a drilling mud plume for several
km. Percent transmittance values,
indicative of fine suspended clay-size
particulates, usually reach background
somewhat further downcurrent than
suspended solids concentrations. The
time required for the drilling mud plume
to be diluted and dispersed to
background levels is 30 to 100 minutes.
Rate of dilution to background of
discharged drilling mud is affected by
the rate of drilling mud discharge,
current speed and turbulence, water
depth and other hydrographic
parameters. Dilution rate can be
controlled somewhat by controlling rate
of mud discharge and discharge pipe
design and position in the water
column.
Virtually all of the drilling mud solids
and some of the soluble components
eventually are deposited on the bottom
under and downcurrent from the
discharge pipe. Maximum drilling mud
accumulation on the bottom usually
occurs a short distance downcurrent
from the discharge. The most useful
tracer of distribution of drilling mud in
bottom sediments is barium. Surficial
sediments (upper 1 cm) up to about 2 km
downcurrent from the mud discharge
may contain elevated concentrations of
barium. Elevated concentrations of
chromium, lead, and zinc may occur in
bottom sediments near the discharge.
Concentrations of these metals in
sediments fall to background con-
centration at a much shorter distance
from the discharge than does sediment
barium concentration.
The major environmental concerns
about discharge of used drilling mudsto
the ocean are that they may be acutely
toxic or cause deleterious sublethal
effects in sensitive organisms and
ecosystems and that heavy metals
associated with drilling muds may be
accumulated by marine organisms to
dangerous concentrations.
A majority of major drilling mud
ingredients are biologically inert or have
a very low order of acute toxicity. Of the
major drilling mud ingredients, only
chrome- and ferrochrome-lignsulfonates
can be considered at all toxic. Their
toxicity is quite low to all but a few
sensitive species (e.g., some corals).
Minor ingredients of some environ-
mental concern include sodium
phosphate salts, detergents, biocides
(chlorinated phenols no longer are
permitted for offshore disposal),
chromate salts and asphalt/oil-based
ingredients. Ordinarily, these materials
are not used in large enough quantities
to cause concern. Their concentrations
should be kept low in drilling muds
destined for ocean disposal. Where
possible, less toxic substitutes should
be used.
Results and Conclusions
To date, the acute toxicity and
sublethal biological effects of more than
20 drilling muds used offshore have
been evaluated with more than 60
species of marine animals from the
Atlantic, Pacific, Gulf of Mexico and
Beaufort Sea. Representatives of five
major animal phyla were tested,
including Chordata, Anthropoda,
Mollusca, Annelida, and Echino-
dermata. Larvae and other early life
stages, and oceanic species (considered
to be more sensitive than adults and
estuarine species to pollutant stress)
were included. In all but a few cases,
acute toxicity, usually measured as 96-
hr. LCso, was 10,000 ppm or higher
drilling mud added. The lowest acute
LCso value was 500 ppm for stage I
larvae of dock shrimp Panda/us danae
exposed to a high density ferrochrome-
lignosulfonate drilling mud from Cook
Inlet, Alaska. Chronic or sublethal
responses were observed i n a few cases
at concentrations as low as 50 ppm.
Field studies of drilling mud plume
dilution and dispersion revealed that
drilling mud concentrations high
enough to cause acute or sublethal
damage to the most sensitive species
and life stages should occur only in the
immediate vicinity of the drilling mud
discharge (to less than 1,000 m
downcurrent) and only for a very brief
time during bulk discharges (generally
less than 2 hours). Therefore, we can
conclude that adverse impacts on water
column organisms of discharge of used
water-based drilling muds to the ocean
should be moderate and of short
duration.
Benthic fauna may be vulnerable to
damage from settling drilling mud and
cuttings solids, through burial or
chemical toxicity. Accumulation of
drilling muds in coarse bottom
sediments may change sediment
texture and thereby affect recruitment
to the benthos of planktonic larvae. At
environmentally realistic levels of
drilling mud in sediment, species
composition of the benthic community
changes toward a more silt/clay-
tolerant assemblage. The species most
sensitive to drilling mud in the water or
sediments appear to be very sensitive
high suspended particulates concentra-
tions. Limited field studies indicate that
recovery of the benthic community
from any effects of discharged drilling
mud is likely to be very rapid (within a
few months).
Heavy metals associated with drilling
muds have a very limited bioavailability
to marine animals. Chromium is the
most bioavailable of the mud-
associated metals. Accumulation from
drilling mud of small amounts of
barium, lead, cadmium and copper was
demonstrated a few times when marine
animals were exposed to high
concentrations of drilling muds or
drilling mud ingredients. Field studies in
the vicinity of drilling mud discharges
have not provided any convincing
evidence of metal accumulation by
resident marinefauna. More research is
needed on the long-term bioavailability
to benthic marine organisms of metals
from sediments comtaminated with
realistic amounts of used drilling muds.
Discharge of used water-based
drilling fluids to the ocean or exposed
coastal waters, where rapid dispersion
and dilution are possible, poses no as
yet measurable hazard of more than
very localized and transitory impact on
the marine environment. Even in the
small temporal and spatial domain in
which an adverse impact can be
observed or predicted, damage is likely
to be of a low order of magnitude and
restricted primarily to the benthos.
Metals associated with used drilling
fluids have a very limited bioavailability
to marine organisms, so there is little
danger of food-web transfer or biomag-
nification of mud-associated metals to
commercial fishery species or Man.
These conclusions apply to standard or
typical water-based drilling fluids
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currently in use for exploratory drilling
in U.S. coastal waters and outer
continental shelf. Higher modified or
specialized mud formulations or
completely new formulations or
ingredients that might be introduced
may behave quite differently in the
marine environment than the majority
of drilling muds evaluated to date.
Jerry M. Neff is with Battelle New England Marine Research Laboratory.
Duxbury, MA 02332.
Thomas M. Duke is the EPA Project Officer (see below).
The complete report, entitled "Fate and Biological Effects of Oil Well Drilling
Fluids in the Marine Environment: A Literature Review," (Order No. PB 82-
240 391; Cost: $16.50, subject to change/ will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Gulf Breeze, FL 32561
> UB. GOVERNMENT PRINTING OFFICE: 1962-559-017/0803
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Environmental Protection Information Fnuirnnmental
Agency Cincinnati OH 45268 Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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